1. Field of the Invention
This invention relates to printed circuit boards widely used in various electronic equipments, and particularly relates to a discharge structure of a printed circuit board for discharging static electricity to a ground pattern when static electricity is generated upon supply of input signals or upon connection of the other component.
The present application is based on Japanese Patent Application No. Hei. 9-213424, which is incorporated herein by reference.
2. Description of the Related Art
In various electronic equipments, for example, in an electronic equipment for car control, miniaturized electronic components or integrated circuits (ICs) are formed on a printed circuit board for making the size compact and making the weight light. Such an IC has a tendency that they may be broken easily by static electricity as disclosed in Unexamined Japanese Patent Publication No. Hei. 3-261087 or the like. For example, an IC mounted on a printed circuit board may be broken by static electricity when a finger or the like touches the printed circuit board, when an input signal is supplied to an input terminal of the printed circuit board, or when other electronic components are connected to the printed circuit board.
Therefore, conventionally, various measures have been taken to cope with generation of static electricity, as follows.
First, referring to FIG. 4, a first example of such a conventional measure against static electricity will be described. Input terminals T21 and T22 are provided on a printed circuit board 31, and electronic circuits 32 and 33 constituted by ICs are connected to the input terminals T21 and T22 through resistors R21 and R22 respectively.
Discharging capacitors C21 and C22 are connected between the input sides of the resistors R21 and R22 and a ground pattern (not shown) respectively.
In such a configuration, static electricity generated at or asserted to the input sides of the resistors R21 and R22 are discharged to the ground through the capacitors C21 and C22 respectively. The capacitors may be replaced by discharging surge absorbers or the like.
Next, referring to FIG. 5, a second example of the measure against static electricity will be described. In a printed circuit board 41, a belt-like ground patten 43 of a copper foil portion is formed on one side surface of an insulating substrate 42, and a resist layer 44 of an insulating material is formed on the ground pattern 43. Insertion holes 45 are formed through the printed circuit board 41, so that connection terminals of connectors or the like are inserted through the insertion holes 45 so as to be connected to a circuit pattern (not shown).
The resist layer 44 is partly removed to form removed portions 46 in positions corresponding to lower portions of components (in the vicinity of input terminals). Accordingly, the ground pattern 43 is partly exposed through the removed portions 46.
Next, referring to FIG. 6, a third example of the measure against static electricity will be described. In a printed circuit board 41, a belt-like ground patten 43 of a copper foil portion is formed on one side surface of an insulating substrate 42, and a resist layer 44 of an insulating material is formed on the surface of the ground pattern 43. Insertion holes 45 are formed through the printed circuit board 41, so that connection terminals of connectors or the like are inserted through the holes 45 so as to be connected to a circuit pattern (not shown).
The resist layer 44 is partly removed to form a belt-like removed portion 47 in a position corresponding to a lower portion of components (in the vicinity of input terminals). Accordingly, the ground pattern 43 is slenderly exposed through the removed portion 46.
In the first example of the discharge measure against static electricity, however, it is necessary to provide components such as the capacitors C21 and C22, surge absorbers, or the like, so that the number of components and the number of work steps increase. Further, static electricity often exhibits a high voltage instantaneously so that the capacitors may be broken by such a high voltage applied thereto. Thus, there is a problem in durability.
On the other hand, in the printed circuit boards 41 illustrated as the second and third examples of the discharge measure against static electricity, the removal portions 40 and 47 are formed to expose the ground pattern to allow discharge. However, since the impedance of the input terminals is low, discharge can not be generated from the input terminals to the ground pattern.
Further, with respect to prevention of breaking of electronic equipments due to static electricity, the above-mentioned Unexamined Japanese Patent Publication No. Hei. 3-261087 discloses "Electrostatic Breaking Preventing Apparatus" and Unexamined Japanese Patent Publication No. Hei. 3-297199 discloses "Electric Apparatus".
In the above "Electrostatic Breaking Preventing Apparatus", a ground pattern is formed on a printed circuit board and a protrusion for accelerating discharge of static electricity is formed at a position of a part of the ground pattern facing an input pattern. However, the input pattern and the ground pattern do not face each other three-dimensionally but only portions of the thin patterns formed on one and the same plane are made adjacent to each other. Accordingly, the discharge effect with respect to the input pattern is not good. In addition, there is a possibility that static electricity due to approach of a finger, a component or the like cannot be discharged.
Further, in the above "Electric Apparatus", an electrostatic come-flying copper foil portion (circuit pattern) is formed on a printed circuit board so that static electricity generated when operating components are mounted is discharged to the electrostatic come-flying foil. However, no consideration is given to discharge of static electricity due to input signal supply.